1. Field of the Invention
The present invention relates to an apparatus configured to measure a distribution of a light absorption coefficient in an organism using the photoacoustic effect, and more particularly to an imaging apparatus of an acoustic image formed by the distribution of the light absorption coefficient that is generated by the photoacoustic effect and the array type (acoustic) detector.
2. Description of the Related Art
The photoacoustic method is a method for irradiating excited light onto a sample and for detecting a periodic volume change of the sample that absorbs the light due to the photoacoustic effect. The photoacoustic method enables information of the light absorption distribution in the organism to be measured and visualized.
The near infrared light is light of a wavelength band (700-1200 nm) referred to as an optical window, and has a longer penetration depth in the organism than light having another wavelength band. The near infrared light is less likely to affect the organism than the electromagnetic wave, such as an X ray. Using the near infrared light having such a characteristic, active research on the measurement and visualization of the information in the organism has recently been conducted. The acoustic wave is as safe to the organism as the near infrared light, and can reach a deep portion in the organism. In addition, the acoustic wave characteristically approximately goes straight in the organism different from the near infrared light. Main ingredients of the organism, such as water, fat, oxygenated hemoglobin, reduced hemoglobin, and collagen has a spectrum characteristic to the light of the near infrared region. It is expected that initial symptoms of a disease, such as a cancer, can be observed through a visualization of the space distribution of the ingredients in the organism. However, the organism has a strong scattering characteristic to the near infrared light, and thus it is difficult to obtain spectrum information of a specific site. The spectrum information of the specific site is available with the photoacoustic method that utilizes the characteristics of the acoustic wave and the near infrared light.
There is known a technology to visualize information in the organism using the photoacoustic method (U.S. Pat. No. 5,840,023).
Since the near infrared light strongly scatters in the organism as described above, the light intensity is small in the deep region in the organism. The intensity of the acoustic wave generated from the specific site in the organism is proportional to the light intensity absorbed in that specific site. Therefore, the intensity of the acoustic wave generated in the deep region in the organism is insufficiently small, and the acoustic method of the U.S. Pat. No. 5,840,023 can obtain only a feeble acoustic wave. Similarly, since a ratio of the wavefront of the acoustic wave generated in the deep region in the organism which enters the acoustic detector reduces for a deeper region in the organism, only a feasible acoustic wave can be obtained.